Net Surface Solar Irradiance Variability in the Central Equatorial Pacific during 1982–1985

Abstract

This paper examines the evolution of the net surface solar irradiance from March 1982 to October 1985 in an important region of the equatorial Pacific where the TROPIC HEAT Experiment took place (4.6°N to 7.4°S, 142.6° to 117.17°W). The investigation, which focuses on the dramatic modification of radiative processes as a consequence of the 1982-83 El Niño episode, analyzes annual and monthly net surface solar irradiance fields computed from geostationary satellite observations. An annual mean for the year October 1984-September 1985, the year most distant from the El Niño event during the study, is computed and compared with existing climatologies. It is found that, while our values compare well with those of Weare et al., they are significantly higher than those of Esbensen and Kushnir and those of Chou. In the case of Esbensen and Kushnir, a climatology often used to force numerical ocean circulation models, the discrepancy reaches 40 W m?2. Among the three annual fields computed, the two non-El Niño years are relatively similar, exhibiting features analogous to those found in all the climatologies. As expected, the El Niño year's annual mean field is remarkably different and is characterized by a complete disappearance of the zonal orientation and much smaller values along the equator, particularly in the western part of the studied region. The monthly mean fields confirm the El Niño's marked effect on net surface solar irradiance, especially during January, February, and March 1983, when the solar irradiance at the equator is reduced by more than 150 Wm?2. An empirical orthogonal function analysis of the monthly fields containing the annual cycle quantifies the annual and interannual variations of the net surface solar irradiance and demonstrates that the two main forcing periods for the study area are both 12 months. The one explaining most of the variance (62%), however, exhibits two pronounced maxima at six-month intervals. This mode is much more perturbed during the El Niño than the other and confirms that the effects of the El Niño on solar irradiance are much stronger in the equatorial region than in higher latitudes; it further indicates that the solar irradiance deviations from the annual mean are more positive (less cloudiness) than during normal years al the end of the event. Although the study's results are limited to the area and the period examined, they are strongly indicative of the radiative processes occurring near the equator during a strong El Niño.